Troubleshooting, from a semi-novice perspective.

By Kevin Morrison

The Symptoms. There aren’t many things that go wrong with the Miata, but when something does go wrong, it’s nice to be able to figure out what’s wrong and take steps to fix the problem yourself. To that end I thought I’d pass along what I’ve been through on the few occasions my Miata has had a problem. Most recently, my Miata started getting really lousy gas mileage (about 20 to 21 miles per gallon). Since I drive mostly on city streets and tend to mash the right-most pedal pretty hard, I thought nothing of it and wrote it off as the effect of a heavy foot. Then one night on a leisurely drive across Tampa Bay, my "check engine" light came on for about five minutes and then went off again. Again, I didn’t think much of it and did nothing about it. A few weeks later, I took a 600-mile trip to my parents’ house in Mississippi and was dismayed to find that my gas mileage on the highway was still only about 21 mpg. A trip that I usually can make on two tanks of gasoline, took almost three tanks instead. Halfway through the drive, the "check engine" light came on for about twenty minutes, and then came on and went off twice more before I reached my destination. Cruising at about eighty miles per hour, I could feel a slight loss of power for a split second that occurred only rarely, and generally when I lifted off the throttle slightly. If it hadn’t been for the check engine light, I would have replaced the plug wires and went on my merry way, but bad plug wires won’t trigger an engine code, so I knew I had to dig deeper.

The diagnosis. For some reason, I had a hunch that my car’s Oxygen sensor was bad. It was the only component I knew of that, if faulty, would give me the combination of symptoms I had observed. The only way for a semi-novice to be able to hazard such guesses is to become familiar with all the systems in the car and know what they do and how they interact. Since the engine computer relies on data from the Oxygen sensor (which is located on the exhaust header just ahead of the catalytic converter) to determine the proper amount of fuel to maintain the right air/fuel ratio, I knew the car would probably run rich if the computer were not getting good data from the O2 sensor. Since there are a few real mechanics in my immediate family (my dad owns Dave’s Garage in Biloxi, Mississippi) I ran my car’s symptoms by them and all agreed that the O2 sensor was the most probable culprit. It wasn’t until later that I learned the way to read my car’s computer error codes using only a paper clip, but it turns out the diagnosis was correct.

Get the Part. Replacing a bad O2 sensor is a pretty straightforward proposition, if you can find the part. The average Miata owner would probably take the car to a Mazda dealership and pay the $214 (plus installation) price for the sensor, which had to be ordered. I couldn’t stomach that, and I only had a couple days to get it fixed before I had to drive back to Florida from Mississippi, so decided to make it a do-it-yourself job. My Dad got on the telephone to his usual parts suppliers and came up completely empty for the O2 sensor for a ’94 Miata. The local Autozone had one to fit a pre-’94, but the sensors are different between those model years. The ‘94+ cars have a 4-wire (heated) sensor, while the early cars have a single-wire sensor. So, I got on the Internet and searched the archives of the Miata Mailing List at Miata.net. I turned up part numbers for two 4-wire oxygen sensors that will fit the Miata. They are:

1. Borg-Warner Part # OS126
2. Bosch Part # 15706

Using these part numbers, my dad got on the telephone with NAPA and was able to locate one of the Borg-Warner units in Dallas. It was shipped overnight and cost us about $50 (list price $175). It’s the exact same sensor the dealership charges $214 for, except it has a wiring harness on it to fit a Dodge Neon. I’ve been told by others that this sensor should be readily available at local auto parts stores. I have no idea why NAPA could not locate one anywhere in the Southeastern US.

Install the Part. The first step is to cut off the wiring harness for the Neon, the Miata does not have a suitable connector for it. Leave a foot or more of the wire connected to the sensor so it will be easier to connect the wires in the engine compartment. Next, remove the old sensor. The best way to do this is with a special slotted socket (22mm). The slot allows you to get the socket on the sensor base with the wire intact. A proper sized open-ended wrench would probably work also. (Note: on the 1.6 cars, the sensor is a little harder to get to.) My car has a Jackson Racing Cold Air Induction, so the exhaust manifold is easy to access, but for a stock car, the airbox will have to be removed to access the sensor from above.

To remove the sensor, one needs a special socket. There a couple different kinds. One is a simple wrench with a gap in it, with a 3/8" socket fitting on the side. It works in very confined spaces, because it is very shallow. Another is a deep socket with a slot in the side to allow the wiring to pass through. The latter type did not work very well on the Miata. It is too large to work in the space next to the header on the 1.8 Miata. Once the old sensor is out, cut it off, leaving as much wire as possible in the car’s harness. Here’s the tricky part. The car’s wiring harness has four wires, one blue, one white and two black. The wires on the Borg-Warner (and the Bosch) sensor are one gray, two white, and one black. I was at a complete loss as to how to connect these wires, so after a long session of thinking, I decided to connect the gray to the blue (sensor output), one black to black (ground), the other black to white (ground?), and the one white to white (+12V). After making the connections and not feeling at all confident about whether they were correct or not, I screwed in the new sensor (22-36 ft-lbs and don’t forget the anti-seize grease) and started the car. It seemed to run better, but after a short test-ride, the check engine light came on again, and stayed on. In fact, the car did run better because instead of getting an intermittent signal from the O2 sensor, it was now getting no signal at all. It was at this point I decided to consult the Internet again and I remembered reading something about reading the engine’s error codes using a paper clip in the diagnostic connector under the hood. Per the instructions at Miata.net, I used a paper clip as a jumper between the pins TEN (Test ENgine) and GND (ground). When so jumpered, the "check engine" light begins to flash its code. First it flashes the tens digit – one long flash means "10", then seven short flashes means "add seven." So my engine error code was "17" which happens to mean "O2 sensor, voltage not changing." So finally I knew the sensor was wired wrong, and there wasn’t some other problem causing the light to come on. So, I reconsidered my wiring. I determined the only other combination of wiring that made any sense (really it didn’t) was to hook black to blue and then connect all the black wires to all the white wires, leaving the gray wire from the sensor to connect to the single white wire on the car. It worked.

Here’s the way to wire it:

OS 126 Sensor wiring                            Car wiring
Black            			Blue (sensor output)
White					Black (heater circuit)
White					Black (heater circuit)
Gray					White (sensor ground)

It takes a leap of faith to go into your car and connect white wires to black wires, but in this case, it’s the right thing to do. A multimeter would have proven invaluable in determining which wire was which, but I didn’t have one at hand. On the way home to Florida, the car got almost 30 miles per gallon, but the "check engine" light came on again. Since I now carry a paper clip in the car at all times, I was able to read the code again at a highway rest stop. It was "15" – "O2 sensor voltage low." I figured one or more of my connections was flaky, since I had run out of crimp connectors and just twisted some of the wires together. The next day I re-crimped all the connections, put some heat-shrink tubing around them, and the engine code did not reoccur. (Well, not quite, I’ll get to that part in the next issue.)

Be Happy. So, even a rank amateur can diagnose and fix a mysterious problem in the Miata. It would be much more difficult for another car that does not have the same level of resources on the Internet. Without it, I would have ended up spending a great deal more money at a dealership and would have been without my car for a day or more, so the payoff for doing it myself was big.